I think you should check out the UK Student Rooms. Most students from the UK choose UCL, LSE, or other programs over the Cambridge course. As to why, I'm not exactly sure. I think 10 months is really pushing it in terms of mastering a subject, especially when you consider a couple of those months are dedicated solely to the dissertation, but that's just me. In the end, it's a degree from Cambridge, so you can skate by on the name itself. With that said, it is without question considered the weakness of the University, and that's unquestionable. You can find many opinions of this on the Cambridge boards and the UK Student Rooms. It's the easiest program to gain admission to, as it isn't immensely popular and most students choose to go elsewhere, like the Unis mentioned up post, as well as Reading, Manchester, etc. I would always recommend never choosing a degree simply for the brand name of the University. In this particular case, there are far better programs out there, but it really depends on what suits your needs.
As to your concerns about it having an adverse effect on getting into PhD programs, most definitely not. It's an MPhil from Cambridge, man! As I said earlier, considering the fact that you plan on going forward, I think in this particular instance, I would choose Cambridge simply for the duration of the program and the fact that it is a strong program, even if not the strongest. You'll get everything you need to get out of it to prepare you for doctorate studies, and from what I've heard of the course, you'll do TONS of reading and writing, more so than you would normally expect from most programs. This is to be expected from Oxbridge, though, which is why their reputation is so sterling. So, I would actually say that while an MSc Spatial Planning from UCL would be more practical if you were looking to go into the field upon graduation, the MPhil Planning, Growth, & Regeneration is more practical if you wish to continue onto PhD studies.